Treatment of animals with inducers of the enzyme system, aryl hydrocarbon hydroxylase (AHH), has been shown to inhibit tumorigenesis induced by benzo(a)pyrene (BP) and other carcinogens. Under the same experimental regime in which beta-naphthoflavone (beta NF) inhibits BP-induced pulmonary tumors, a good correlation was found between the decrease in the amount of (plus)-BP-7 beta, 8 alpha-diol-9 alpha, 10 alpha-epoxide-DNA adduct (BPDE-I adduct) and inhibition in pulmonary adenomas. Formation of the diol epoxide I adduct has been suggested to be a necessary event in tumor initiation. In mice pretreated with 2,3,7,8-tetrachlorodibenzodioxin (TCDD), another AHH inducer, the BPDE-I adduct was also decreased by more than 90 percent. These results suggest that treatment of animals with AHH inducers protects against BP-induced carcinogenesis by inhibiting the formation of BPDE I-adduct. Treatment of rats with beta-NF or TCDD also decreased the BP-nucleoside adduct in lung and liver by more than 90 percent.